Photochromic glasses darken under the influence of actinic radiation, commonly ultraviolet radiation. They return to their original state when the radiation is removed. Such glasses are developed by thermal treatment to produce crystallites of a silver halide selected from the group consisting of AgCl, AgBr and AgI, in the glass.
Glasses demonstrating photochromic behavior were originally described in U.S. Pat. No. 3,208,860 (Armistead et al.). As there described, photochromic glasses were produced in a R.sub.2 O--Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 --SiO.sub.2 base glass system. The base glass consisted essentially of 4-26% Al.sub.2 O.sub.3, 4-26% B.sub.2 O.sub.3, 40-76% SiO.sub.2, and R.sub.2 O, the R.sub.2 O being selected from the group consisting of 2-8% Li.sub.2 O, 4-15% Na.sub.2 O, 6-20% K.sub.2 O, 8-25% Rb.sub.2 O, and 10-30% Cs.sub.2 O, the total of these basic ingredients being at least 85%. To provide photochromic properties, the glass contained at least one halide in a minimum amount of 0.2% Cl, 0.1% Br, and 0.08% I, and silver in a minimum amount of 0.2%, 0.05% and 0.03% where the added halide is, respectively, Cl, Br, or I.
Subsequent to that basic disclosure, extensive research efforts have been made to continually develop glasses having improved photochromic properties. These efforts are evidenced by the considerable volume of patent and other literature that has been generated. Most of the research has been focused on the ophthalmic field to provide both prescription lenses and non-prescription sunglasses. Accordingly, primary interest has been in glasses that darken rapidly to a moderately low luminous transmittance under the influence of an exciting radiation, and then fade rapidly to the original transmittance when removed from the exciting radiation.
For example, darkening levels were sought ranging from about 15 to 25 percent transmittance for sunglasses to about 40 to 50 percent transmittance for comfort glasses. At the same time, ever faster fading rates were also sought. For example, a fading rate was desired such that the glass regained half its original transmittance in five minutes. It was further deemed desirable to retain rapid darkening and fading rates over the range of temperatures customarily encountered by one wearing the glasses.
A current photographic processing application requires a very different photochromic performance. This process uses a flat plate of photochromic glass as a temporary filter. The filter creates an enhanced contrast, mask image of a photographic slide or negative that may be either black and white or color. The procedure greatly enhances the quality of the image made from an original. It does this by printing the subtle tones in dark areas of negatives while attenuating the light energy through highly transparent areas.
The requirements for this application are:
1. Darkening to a transmittance below about 25 percent, preferably not over 20%. PA1 2. Darkened color is preferably neutral grey. PA1 3. The glass should stay darkened for several minutes when removed from the activating light. PA1 4. The glass must fade back to the clear state once the printing process is completed. A heat fade can be used. PA1 a. a clear luminous transmittance of about 90%, PA1 b. a darkened luminous transmittance at 20.degree. C below about 25%, preferably not over 20%. PA1 c. a fading rate at 20 C such that, after a five minute fading interval, the glass exhibits a percent faded luminous transmittance that is no more than the darkened luminous transmittance percentage plus 15 percentage units, PA1 a. a clear luminous transmittance of about 90%, PA1 b. a darkened luminous transmittance at 20.degree. C. below about 25%, preferably not over 20%. PA1 c. a fading rate at 20.degree. C. such that, after a five minute fading interval, the glass exhibits a percent faded luminous transmittance that is no more than the darkened luminous transmittance percentage plus 15 percentage units.
It is a basic object of our invention to provide photochromic glasses that rapidly darken to a very low transmittance, that fade back very slowly under ambient conditions, but may be bleached by special treatment such as heating.
Another purpose is to provide novel photochromic glasses having characteristics that particularly adapt them to use in the photographic processing application described above.